Umbilical cord blood has been increasingly used as an alternative source of stem cells and has extended the availability of allogeneic HSCT to patients who would otherwise not be eligible for this curative approach. In the last decade the number of cord blood transplantations from related and unrelated donors has increased dramatically. It is estimated that more than 20,000 patients have undergone cord blood transplantation from unrelated donors to date for a variety of genetic, hematological, immunological, metabolic and oncologic disorders. The major advantages of cord blood transplantation include easy procurement, no risk to donors, reduced incidence of transmitting infections, immediate availability, and reduced risk of acute GvHD in the setting of donor-recipient HLA mismatch. Nevertheless, the low cell dose remains a main limitation of this cell source leading to delayed hematopoietic reconstitution, higher risk of graft failure and relatively high treatment related mortality rates as compared to other hematopoeitic cell sources. To improve outcomes and extend applicability of cord blood transplantation, one potential solution is ex vivo expansion of cord blood-derived stem and progenitor cells.